This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP99/02729 which has an International filing date of May 25, 1999, which designated the United States of America.
1. Field of the Invention
The present invention relates to an anode for a secondary battery using, as an electrode material, a sintered material which contains silicon as an active material, a process for producing the same, and a non-aqueous secondary battery using the same.
2. Description of the Related Art
With the popularization of portable telephones and note-type personal computers, high-capacity lithium secondary batteries containing a cathode active material and an anode active material, capable of incorporating and releasing lithium ions, have attracted special interest. Among them, demands for space-saved, thin square-shaped batteries have been particularly enhanced. For the purpose of enhancing the efficiency of the battery reaction by increasing the electrode area, a cathode and an anode obtained by coating a belt-shaped metal foil with a coating composition containing an active material, a binder and a conductive material are used in a current square-shaped battery, and these electrodes are contained in a battery can after winding, together with a separator, and pressing.
This electrode is composed of about 40% by volume of an active material, 20 to 30% by volume of a binder, a conductive material and a metal foil, and 30 to 40% by volume of pores. Accordingly, there is a problem that those which do not contribute intrinsically to the capacity of the battery, such as binder, conductive material and metal foil limit the battery capacity per volume. When the wound electrodes described above are contained in the square-shaped can, it is impossible to fill corner portions of the battery and a useless space is formed. Therefore, the capacity per unit volume is further lowered.
Thus, a trial of forming the electrode of a sintered material made substantially of an active material has been made as a means for increasing the capacity per unit volume. When the electrode is formed of the sintered material, it is possible to eliminate the binder and to eliminate the conductive material or to reduce its amount, thereby making it possible to increase the filling density of the active material and to enhance the capacity per unit volume. For example, Japanese Patent Laid-Open Publication No. 5-299090 discloses an anode obtained by contact-bonding of a copper foil to a sintered material of a petroleum pitch or a carbonaceous material, while Japanese Patent Laid-Open Publication No. 8-180904 discloses a cathode formed of a sintered material of a composite oxide containing lithium and metal.
As the anode active material, carbon materials, for example, amorphous carbon such as coke (e.g. Japanese Patent Laid-Open Publication No. 62-122066 and 1-204361) and glassy carbon (e.g. Japanese Patent Laid-Open Publication No. 2-66856); and natural graphite (e.g. Japanese Patent Publication No. 62-23433) or artificial graphite (e.g. Japanese Patent Laid-Open Publication No. 4-190555) have been suggested. However, the battery capacity per unit volume is not sufficient even in case where any of amorphous and crystalline carbon materials is used and, therefore, a further improvement in performance is desired.
To increase the battery capacity per unit volume, a trial of using silicon or its compound as the anode active material to form an anode has been made. For example, Japanese Patent Laid-Open Publication No. 7-29602 discloses a process of producing an anode, which contains using LixSi (0xe2x89xa6xc3x97xe2x89xa65) as an anode active material, adding graphite as a conductive material and a binder, forming the mixture into pellets and using a conductive adhesive as a current collector. Japanese Patent Laid-Open Publication No. 5-74463 discloses a process of producing an anode, which contains using a silicon single crystal as an active material and interposing it between nickel meshes.
However, even if an anode which contains silicon as the active material is formed of a sintered material so as to increase the capacity per unit volume, the internal resistance of the battery is increased by a large contact resistance between a current collector and a sintered material, whereby the capacity is not improved necessarily at present.
In view of the requisite capacity in case of using in the portable telephone, the base area of the electrode is preferably 4 cm2 or more because of limitation of the thickness of the battery.
However, in the anode sintered material composed mainly of silicon, those satisfying these requirements simultaneously could have not obtained by a conventionally known technique.
It is, therefore, an object of the present invention to provide a process for producing an anode for a secondary battery, capable of reducing contact resistance between a current collector and a sintered material in an anode which contains silicon as an active material.
By sintering a silicon-containing coated film and a base material made of a foil or mesh of conductive material, a high capacity can be obtained. That is, the process for producing an anode for the secondary battery of the present invention comprises: (a) adding a binder and a solvent to a silicon-containing anode material to prepare a slurry; (b) coating a base material made of a foil or mesh of conductive metal with the slurry, and then removing the solvent to form a coated film; and (c) sintering the coated film in a non-oxidizing atmosphere, thereby integrating a sintered material of the coated film with the base material.
By sintering a silicon-containing coated film and a base material made of conductive metal in a non-oxidizing atmosphere, the contact area of interface between a sintered material and a current collector are increased, and the sintered material is integrated with the current collector, thereby making it possible to reduce the contact resistance between the sintered material and the current collector and to provide an anode of a thin film whose conductivity has been improved.
The anode material preferably contains a material to be carbonized by a heat treatment, or a carbon material. In that case, it is preferred to use a composite powder obtained by heat-treating silicon or its compound at a temperature within a range from 600 to 1400xc2x0 C. in a non-oxidizing atmosphere in the presence of a material to be carbonized by the heat treatment, or a carbon material.
The coated film is preferably sintered at a temperature lower than the melting point of conductive metal base material, thereby making it possible to integrate the sintered material with the base material without causing thermal deformation of the base material.
By using any one metal selected from stainless steel, elements of the upper group and elements of the platinum group as the conductive metal, there can be obtained a current collector which is electrochemically stable even in a reduced state of the anode and has high conductivity.
The anode for the secondary battery according to the present invention is characterized by an anode obtained by the process of sintering a coated film which is formed on a base material made of a foil or mesh of conductive metal and comprises a silicon-containing anode material and a binder, thereby to integrate the anode sintered material with the base material.
The anode for secondary battery according to the present invention can also be obtained by peeling a coated film from a base film for coating, pressing the coated film to a base material made of a foil or mesh of it conductive metal, and sintering them, thereby integrating a sintered material of the coated film with the base material. When using the base film for coating, it also becomes possible to continuously process drying of a coated film coated with the slurry, peeling of the coated film and recovery of the coated film, using a coated film winder, thereby simplifying the production process.
The thickness of the anode sintered material is preferably in a range of 10 to 500 xcexcm, and the base area is preferably 4 cm2 or more. Furthermore, the sintered material preferably contains 30 to 90% by weight of silicon and 10 to 70% by weight of a carbon material.
The non-aqueous secondary battery of the present invention comprises an anode obtained by sintering a coated film comprising a silicon-containing anode material, a binder and a base material made of a foil or net of conductive material, thereby integrating a sintered material of the coated film with the base material; a cathode made of a lithium transition metal oxide as an effective ingredient; and an electrolyte obtained by dissolving a lithium compound in an organic solvent, or a solid electrolyte containing a lithium ion-conductive non-aqueous electrolyte, the solid electrolyte is obtained by incorporating a lithium compound into a polymer in a solid state or retaining the organic solvent containing the lithium compound dissolved therein with the polymer. It is preferred to use a sintered material made of a lithium transition metal oxide as a cathode.
The non-aqueous secondary battery of the present invention is preferably subjected to an electrochemical charge and discharge treatment. Not only charging/discharging at a high current density can be conducted, but also a high capacity can be obtained.
This application is based on application No.10-142960 filed May 25, 1998 in Japan, the content of which is incorporated hereinto by reference.
The anode used in the present invention contains silicon as the anode active material. The silicon powder used in the present invention may be any of amorphous and crystalline of elemental substance of silicon. Furthermore, the silicon compound which can be converted into silicon by decomposing or reducing in a non-oxidizing atmosphere, for example, inorganic silicon compound such as silicon oxide; and organic silicon compound such as silicone resin and silicon-containing polymer compound, can be used in the present invention. Among them, the elemental substance of silicon is particularly preferred. The purity of the silicon powder is not specifically limited, however, the silicon content is preferably 90% by weight or more so as to obtain a sufficient capacity, and is preferably 99.999% by weight or less in view of economical efficiency. The particle diameter of the silicon powder is not specifically limited, however, those having an average particle diameter of 0.01 to 100 xcexcm are preferably used in view of handling, cost of raw material and uniformity of the anode material.
The anode material used in the present invention is preferably a composite powder including the carbon material. The composite powder is made by subjecting silicon or its compound to a heat treatment in a non-oxidizing atmosphere at a temperature within a range where silicon is not molten and sufficient sintering can be conducted, for example, 600 to 1400xc2x0 C., preferably 800 to 1200xc2x0 C. in the presence of a carbon material or a material to be carbonized by the heat treatment. The carbon material used herein includes, for example, coke, glassy carbon, graphite, carbonized pitch, and a mixture thereof.
The material to be carbonized by the heat treatment includes, for example, thermosetting resin such as phenol resin, epoxy resin, unsaturated polyester resin, furan resin, urea resin, melamine resin, alkyd resin, and xylene resin; condensed polycyclic hydrocarbon compound or its derivative, such as naphthalene, acenaphthylene, phenanthrene, anthracene, triphenylene, pyrene, chrysene, naphthacene, picene, perylene, pentaphene, and pentacene; or pitch containing a mixture of the above condensed polycyclic hydrocarbon compound or its derivative compound as a main component. Among them, pitch is preferred.
The conductive metal used as the base material may be any one metal selected from stainless steel, elements of the copper group and elements of the platinum group, but is preferably copper which is easily reduced, and has high electric conductivity and cheap price. As the conductive metal, a foil or mesh may be used but the thickness is preferably from 3 to 100 xcexcm.
The base film for coating may be any one which has smooth surface and is capable of peeling a coated film, and there can be used a polymer film of polyethylene, polypropylene, polyethylene terephthalate, and polyethylene naphthalate. These films are preferably subjected to a peeling treatment. The thickness is preferably from 3 to 100 xcexcm.
When the conductive base material or base film for coating is coated with the anode material, a publicly known binder dissolved in a suitable solvent such as water and N-methyl-2-pyrrolidone can be used. As the solvent, any of aqueous and non-aqueous solvents may be used. As such a binder, for example, there can be used any conventionally known materials such as polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyvinyl alcohol, and polyvinyl pyrrolidone.
The temperature at which the anode material to be sintered is preferably lower than the melting point of the conductive metal to be used. When using copper, the temperature is lower than the melting point of copper 1083xc2x0 C., and preferably from 500 to 900xc2x0 C. The sintering of the anode coated film and the sintering for producing a composite powder which contains silicon and a carbon material can be combined.
The thickness of the anode sintered material is preferably 10 xcexcm or more in view of the strength, and is preferably 500 xcexcm or less in view of the performance at a high current density.
On formation of the battery, the base area of the anode sintered material is preferably 4 cm2 or more so as to realize easy handling.
The sintered material of the present invention is preferably a porous material having a porosity of 15 to 60% so that the electrolyte is sufficiently made contact with the active material.
The cathode active material in the present invention may be any conventionally known material, and examples thereof include LixCoO2, LixNiO2, MnO2, LixMnO2, LixMn2O4, LixMn2xe2x88x92yO4, xcex1-V2O5, and TiS2.
The non-aqueous electrolyte used in the present invention may be a non-aqueous liquid electrolyte dissolving a lithium compound in an organic solvent, or a polymer solid electrolyte obtained by incorporating a lithium compound into a polymer in a solid state or retaining the organic solvent containing the lithium compound dissolved therein with the polymer. The non-aqueous liquid electrolyte is prepared by appropriately combining an organic solvent with an electrolyte, and the organic solvent and electrolyte may be any one which can be used in this kind of the battery. The organic solvent includes, for example, propylene carbonate, ethylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methylethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethanemethylformate, butyrolactone, tetrahydrofuran, 2-metyhyltetrahydrofuran, 1,3-dioxorane, 4-methyl-1,3-dioxofuran, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, 1,2-dichloroethane, 4-methyl-2-pentanone, 1,4-dioxane, anisole, diglyme, dimethylformamide and dimethyl sulfoxide. These solvents can be used alone, or two or more kinds of them can also be used in combination.
The electrolyte includes, for example, LiClO4, LiAsF6, LiPF6, LiBF4, LiB(C6H5)4, LiCl, LiBr, LiI, LiCH3SO3, LiCF3SO3, and LiAlCl4. These electrolytes can be used alone, or two or more kinds of them can also be used in combination.
The polymer solid electrolyte used in the present invention may be those obtained by incorporating an electrolyte selected from the above electrolytes into a polymer described below in a solid state. The polymer includes, for example, polymer having a polyether chain, such as polyethylene oxide and polypropylene oxide; polymer having a polyester chain, such as polystyrene succinate and polycaprolactam; polymer having a polyamine chain, such as polyethyleneimine; and polymer having a polysulfide chain, such as polyalkylenen sulfide. The polymer solid electrolyte used in the present invention may be plasticized polymer obtained by supporting the above non-aqueous liquid electrolyte with a polymer such as polyvinylidene fluoride, vinylidene fluoride-tetrafluoroethylene copolymer, polyethylene oxide, polyacrylonitrile, and polypropylene oxide.
The battery thus assembled using the above members is characterized by passing through the process of charging/discharging at a low current (aging process), thereby enabling the battery to exert charge/discharge at a high current density and high capacity. The reason why the charge/discharge efficiency at a high current density of the battery produced without passing through the process is low is assumed as follows. That is, the reaction with such a structural change that crystalline silicon is converted into amorphous silicon by incorporating lithium into crystalline silicon is too slow to follow up the charge/discharge reaction at high current density.